Field of the Invention
The present invention relates to a dot printer designed to form a picture by aggregating dots on a recording medium and, more particularly, to an ink dot printer of a type which deposits ink on a recording medium by sputtering droplets of the ink thereto through electrostatic means.
In the known ink jet printer for forming a picture on a recording medium by jetting droplets of liquid ink thereto from a nozzle, there have been contrived a variety of inventions and improvements heretofore due to some advantages including reduced printing noise and lower running cost in multicolor printing as compared with other printing systems. However, there exists a problem in practical use that no complete measure is achievable to prevent clogging of the nozzle with ink caused by the evaporation of the ink.
In an attempt to eliminate such clogging of the nozzle in the type using liquid ink, there has been developed a further improved system different from the above ink jet printer.
In some examples of such system, as disclosed in Japanese laid-open patent Nos. 56-170 and 56-4467, the nozzle prone to be clogged is replaced with a slit-like opening formed to hold the ink therein, and an electric field is applied between the opening and an opposed electrode so as to sputter the ink electrostatically. In this system, however, the structure for releasing ink droplets from the slit-like opening is delicate and lacks stability. For ensuring a stable printing operation, it is necessary to keep the slit and the recording paper in the mutual proximity with a gap of 100 to 200 microns, but the recording paper is apt to be soiled with the ink held in the slit, and simultaneously dropout of some dots is also liable to occur.
In another type, as disclosed in laid-open U.S. Pat. Nos. 54-23534 and 59-159355, magnetic ink is introduced by magnetic means to the fore end of a needle-shaped member along its periphery and then is sputtered therefrom electrostatically. However, in this type also, there exists a drawback that the use of magnetic ink diminishes the degree of freedom in selecting a desired color since the chromatic choice is restricted by the inherent ground color of the magnetic powder contained in the magnetic ink.
Under the circumstances mentioned above, the present applicants previously invented an improved ink dot printer of the type shown in FIG. 10 and filed an application for patent. In such improvement, first an ink receptacle 2 is disposed for storing ordinary liquid ink therein, and a needle-shaped recording electrode 3 of a conductive and ink-impregnable material is positioned in the ink receptacle 2 while being immersed in the ink 1 in such a manner that the fore end 4 of the recording electrode 3 projects from the ink receptacle 2. And an opposed electrode 6 is placed opposite to the fore end 4 of the recording electrode 3 through a recording paper 5. Furthermore, power sources 8 and 9 for applying a DC voltage via a switching circuit 7 are connected between the recording electrode 3 and the opposed electrode 6. And a printing controlled circuit 10 for generating a control signal in accordance with a picture signal is connected to the switching circuit 7.
The recording electrode 3 employed in the above printer consists of:
(1) a member produced by casting metallic grains while retaining air permeability therein;
(2) a member produced by sintering conductive grains such as transition metal oxide; or
(3) an air-permeable foamed member molded of a conductive plastic material with carbon particles mixed therein.
Out of such three members, each of (1) and (2) is an aggregation of amorphous grains where holes are formed among them and communicate with one another three-dimensionally, and (3) also has a similar structure. Therefore the in 1 is introduced continuously to the fore end 4 of the recording electrode 3. When the switching circuit 7 is actuated in response to a picture signal, an electric field is generated between the recording electrode 3 and the opposed electrode 6 to sputter the ink 1 from the fore end 4 toward the recording paper 5. In this stage of operation, the electric field is easily concentrated on the fore end of the recording electrode 3, so that the ink 1 can be properly sputtered therefrom to stabilize the printing state. Consequently it becomes possible to dispose the recording electrode 3 and the recording paper 5 at mutually spaced positions with a sufficient clearance to eventually prevent soil of the recording paper or dropout of some dots during the printing. Furthermore, a multicolor printing mode can be effected with facility due to the nonnecessity of using any magnetic ink.
Thus the ink dot printer shown in FIG. 10 has accomplished amazing progress in the conventional technology, but in order to achieve a higher printing speed, it is necessary to further increase the amount of supplied ink 1 without the problem of dripping the ink from the fore end 4 of the recording electrode 3. In other words, the recording electrode 3 needs to be capable of holding the ink with certainty while still having a small fluid resistance.